This invention relates generally to the field of radio-frequency (RF) identification technology. More specifically, We invention relates to chipless R tags.
Nowadays, the term Radio Frequency Identification (RFID) covers small data-carrying devices that can be sensed at a distance by RIF receivers. The RID devices typically transmit RF radiation having a frequency of between 100 Hz to 10 GHz. Radio frequency identification tags and radio frequency identification tag systems are known, and find numerous uses. For example, radio frequency identification tags are frequently used for either personal identification in automated gates protecting secured buildings or areas, or for identification and/or security marking of articles such as bank-notes, cheques, credit cards, passports, packaging and tickets.
Today there are mainly two kinds of RF tags: tags that involve a microchip and are termed chip tags and newer, less known, chipless tags. Low cost REID refers to tags below one dollar each, where in chip tags the chip is the largest item of cost. By contrast, chipless tags have much lower prices, are thinner, more physically flexible than chip tags, they work over a wider temperature range and are less susceptible to electrical interference. However, despite all these advantages, the chipless tags known today usually have limited memory and no encryption, and the capability of such systems for identification of a each single item when the items are piled is inferior in comparison to chip tag Systems.
Examples of identification methods include the incorporation of holographic images in plastic credit/charge cards and xe2x80x9csmart cardsxe2x80x9d which incorporate electronic circuitry on microchips. It is also known to mark articles with phosphorescent and/or fluorescent materials, which emit light of a particular wavelength, usually in the visible spectrum, when illuminated by excitation radiation. GB 2 232 119 describes a method for identification and/or security marking which involves the use of triboluminescent phosphors. Triboluminescent phosphors are materials that emit light as a result of mechanical action, for example frictional forces or pressure. Thus, the energy stimulation to produce the phosphorescent emission of light is provided in these compounds by mechanical force. The emission of this radiation serves to identify or confirm the authenticity of a test article and it is detected by a light detector.
Another class of light emitting materials are those involved in the construction of light emitting diodes. Organic light emitting diodes (OLED) refers to a type of display which uses organic material as a diode type light emitting material. OLEDs operate on the principle of converting electrical energy into light, a phenomenon known as electroluminescence. They exploit the properties of certain organic materials which emit light when an electric current passes through them. In its simplest form, an OLED consists of a layer of his luminescent material sandwiched between two electrodes. When an electric current is passed between the electrodes, through the organic layer, light is emitted with a color that depends on the particular material used. In order to observe the light emitted by an OLED, at least one of the electrodes must be transparent
There are two categories of OLEDs; small molecule and polymer-based. When a current is passed through a cell, the polymers molecular structure is excited, creating light emission. The output efficiency of this process is within the spectrum from blue to infrared. Also, by blending polymers with different emission and charge-transport characteristics, OLEDs can be fabricated in which the emission colour varies as a function of the operating voltage.
Voltage sensitive dyes (VSD) have been used extensively for a decade by neurobiologists and cardiologists for following the propagation of nerve signals, employing imaging techniques. However, the use of these compounds has not been extended over the life science field.
Transmission and fluorescence signals of voltage-sensitive molecular probes have provided a powerful method for measuring changes in membrane potential (A. Grinvald et. al., Biophys. J. Biophysical Society, 39, 301-308, 1982). The VSD molecules bind to the external surface of excitable membrane indicated as molecular transducer that transforms changes in the membrane potential into optical changes. These optical signals are correlated with the membrane potential changes and are monitored with light imaging devices such as fast cameras or with a diode array camera.
According to one aspect of the present invention there is provided a method of marking an article to enable its identification, the method comprising: applying to the article a conductive path, which is at least partially coated with a layer comprising at least one compound selected from voltage sensitive fluorescent dye (VSF), mixture of such dyes, electro luminescent (EL) compound, OLED compound or mixtures thereof Such layer is hereinafter termed xe2x80x9cdye layerxe2x80x9d and compounds of the type of VSFD, EL and OLED are hereinafter referred to as xe2x80x9cdyesxe2x80x9d. In addition, a compound termed in the present application xe2x80x9cOLED compoundxe2x80x9d, is a diode type light emitting organic material.
The article may be subjected to means for stimulating fluorescence from said dye layer and when subjected thereto, the fluorescent radiation from the dye layer is transferred into an electrical signal, which serves to ides or confirm the authenticity of the article.
In a preferred embodiment, a plurality of conductive paths is applied to the article, each conductive path being at least partially coated with a dye layer comprising at least one compound selected from a voltage sensitive fluorescent dye (VSFD), mixture of such dyes, electro luminescent (EL) compound, OLED compound or mixtures thereof, each of the dye layers being individually excitable to cause fluorescent radiation emitted from each of said VSFD layers, thereby producing a plurality of signals indicative of the fluorescent radiation emitted by the dye layers which serve to identify the article.
According to another aspect of the present invention there is provided a radio frequency (RF) tag comprising a substrate, a conductive path at least partially coated by a dye layer comprising at least one compound selected from a voltage sensitive fluorescent dye (VSFD), mixture of such dyes, electro luminescent (EL) compound, OLED compound or mixtures thereof, a photosensitive means, and a radio frequency antenna, wherein:
the dye layer is capable of emitting light in response to a voltage signal induced in the conductive path;
the photosensitive means is capable of receiving the emitted light and converting it to an electrical signal that depends on the intensity of the emitted light; and
the RF antenna is electrically connected to the photosensitive means, so as to be responsive to the electrical signal, produced in the photosensitive means in response to the light emitted from the dye layer, to transfer the electrical signal into an RE signal.
According to a preferred embodiment of this aspect of the invention, the tag includes a plurality of interconnected conductive paths rather than a single one. Each of these conductive paths at least partially incorporates a dye layer, so that a voltage signal induced in any one of the conductive pats causes the dye layer involved in this path to emit light The plurality of light emissions emitted from the plurality of dye layers are transferred by the light sensitive means to a plurality of electrical signals, and these are transmitted by the antenna as a plurality of RF signals.
Preferably, the tag according to this embodiment is arranged to transmit the plurality of RE signals sequentially. This may be obtained, for instance, by fabricating the inter-path electrical connections from a material having a low electrical conductivity or by a delay line. The low conductivity of the inter-path connections or the delay line causes the voltage signal induced in one conductive path in tie sequence to reach the next conductive path in the sequence after a time delay, which is set to be long enough so that the light sensitive means will treat the emission from each of the dye layers as an independent signal.
A further aspect of the present invention is an article carrying a tag according to the invention. In particular, it may sometimes be desired that the substrate of the tag is the article itself
One preferable method for fabricating a tag according to the invention comprises the following steps:
(i) providing a substrate;
(ii) printing on the substrate at least one conductive path;
(iii) printing at least partially on each of the said at least one conductive path a dye layer comprising a voltage sensitive fluorescent dye (VSFD), a mixture of such dyes, an EL or OLED compound or mixtures thereof, thus forming at least one printed conductive path at least partially coated;
thus forming at least one printed conductive path at least partially coated;
(iv)applying to said at least one printed conductive path formed in step (iii) photosensitive means that is capable of sensing light emitted from said dye layer coating said conductive path; and
(v)printing an antenna electrically connected to the photosensitive means and to one of the conductive paths.
Preferably, the tag is finally coated with a protective lacquer.
The printing in any one of the steps (ii), (iii), and (v) may be carried out by any kind of known printing method, for example ink jet printing, offset printing impression printing, flexo printing, screen printing gravure printing, laser toner printing, thermal printing and thermal transfer printing. The inkjet printing method is considered favorable.